1. Field of the Invention
The present invention relates to a quinoxaline derivative. In addition, the present invention relates to a light-emitting element, a light-emitting device, and an electronic device each of which uses the quinoxaline derivative.
2. Description of the Related Art
An organic compound can take various structures compared with an inorganic compound, and it is possible to synthesize a material having various functions by appropriate molecular design of an organic compound. Owing to these advantages, photo electronics and electronics which utilize functional organic materials have been attracting attention in recent years.
For example, a solar cell, a light-emitting element, an organic transistor, and the like exemplify an electronic device using an organic compound as a functional material. These devices take advantage of electric properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been making remarkable progress.
It is considered that a light emission mechanism of a light-emitting element is as follows: when voltage is applied between a pair of electrodes which interpose a light-emitting layer, electrons injected from a cathode and holes injected from an anode are recombined in the light emission center of the light-emitting layer to form a molecular exciton, and when the molecular exciton relaxes to the ground state, energy is released and light is generated. As the excited state, a singlet excited state and a triplet excited state are known, and it is considered that light emission can be obtained through either of the excited states.
In improving element characteristics of such a light-emitting element, there are a lot of problems depending on a material, and in order to solve the problems, improvement of an element structure, development of a material, and the like have been carried out.
For example, as a material with an electron-transporting property for a light-emitting element, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) is widely used (see Non-Patent Document 1). However, development of a material with more superior characteristics such as further higher mobility has been demanded. In particular, in view of commercialization, less power consumption is an important object, and developments of the material and the light-emitting element with more superior characteristics have been desired.    [Non-Patent Document 1] Taishi TSUJI, et al., SID 04 DIGEST, 35, PP. 900-903 (2004).